The present invention relates generally to a zoom lens, and more particularly to a high-aperture-ratio, high zoom-ratio zoom lens system including a wide-angle zone which has a phototaking field angle of at least 70xc2x0 suitable for cameras in general, and video cameras or digital cameras in particular.
In recent years, attention has been paid on digital cameras (electronic cameras) which are potential next-generation cameras superseding silver-salt 135 mm film (usually called Leica size) cameras. For digital cameras for general users, single-focus lenses having a diagonal field angle of about 60xc2x0 or zoom lenses of about 3 magnifications using the same at wide-angle ends go mainstream. For high-class users, on the other hand, zoom lenses must be further extended to the wide-angle or telephoto end, and be compatible with TTL optical finders as well. As a matter of course, such zoom lenses are required to have ever higher performance. For zoom lenses having a diagonal field angle of about 75xc2x0 at the wide-angle end and about 7 to 10 magnifications and compatible with TTL optical finders, some are now commercially available for the aforesaid silver-salt 135 mm film cameras. However, wide-angle, high-zoom-ratio zoom lenses, which are well suitable for image-pickup formats considerably smaller in size than the film camera formats and are fast as expressed by an F-number of about 2.0 to 2.8 at the wide-angle end, are little known except those for TV cameras and other commercial purposes.
The state of the art being like this, an object of the present invention is to provide a wide-angle, high-zoom-ratio zoom lens, and especially a zoom lens system which is compatible with a TTL optical finder having a diagonal field angle of at least 70xc2x0 at the wide-angle end and about 7 to 10 magnifications, and is fast as well, as expressed by an F-number of about 2.0 to 2.8 at the wide-angle end.
To achieve this object, the present invention basically provides
a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of the zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of the zoom lens system along the optical axis during zooming from a wide-angle end to a telephoto end of the zoom lens system and has negative refracting power, and a rear lens group having at least two movable subgroups or, alternatively,
a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of the zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of the zoom lens system along the optical axis during zooming from a wide-angle end to a telephoto end of the zoom lens system and has negative refracting power, and a rear group which is located subsequent to the second lens group and has at least two spacings variable during zooming.
Such constructions are favorable for achieving high zoom ratios while various aberrations are minimized. The present invention having such basic constructions has the following characteristic features.
According to the first embodiment of the present invention, there is provided a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of the zoom lens system during zooming and having positive refracting power, a second lens group which moves toward an image side of the zoom lens system along the optical axis during zooming from a wide-angle end to a telephoto end of the zoom lens system and a rear lens group having at least two spacings variable during zooming, wherein a focal length f1 of the first lens group satisfies the following condition (1):
6 less than f1/L less than 20xe2x80x83xe2x80x83(1) 
where L is a diagonal length of an effective image pickup surface located in the vicinity of an image-formation plane.
When the lower limit of 6 to condition (1) is not reached, spherical aberrations remain under-corrected at the telephoto end. When the upper limit to 20 is exceeded, the amount of zooming movement of the movable groups increases, and so the overall size of the zoom lens system tends to increase.
More preferably, condition (1) should be reduced to
6.5 less than f1/L less than 16xe2x80x83xe2x80x83(1xe2x80x2) 
Most preferably, condition (1) should be reduced to
7 less than f1/L less than 12xe2x80x83xe2x80x83(1xe2x80x3) 
According to the second embodiment of the present invention, there is provided a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of the zoom lens system during zooming and having positive refracting power, a second lens group which moves toward an image side of the zoom lens system along the optical axis during zooming from a wide-angle end to a telephoto end of the zoom lens system and a rear lens group having at least two movable subgroups or a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of the zoom lens system during zooming and having positive refracting power, a second lens group which moves toward an image side of the zoom lens system along the optical axis during zooming from a wide-angle end to a telephoto end of the zoom lens system and a rear lens group having at least two spacings variable during zooming, wherein a focal length f1 of the first lens group and anomalous dispersion xcex94xcex8gF of a medium of at least one positive lens in the first lens group satisfy the following conditions:
6 less than f1/L less than 20xe2x80x83xe2x80x83(1) 
0.015 less than xcex94xcex8gF less than 0.1xe2x80x83xe2x80x83(2) 
where L is a diagonal length of an effective image pickup surface located in the vicinity of an image-formation plane.
It is here noted that the anomalous dispersion xcex94xcex8gF of each medium (vitreous material) is defined by
xcex8gF=AgF+BgFxc2x7xcexdd+xcex94xcex8gF 
with the proviso that xcex8gF=(ngxe2x88x92nF)/(nFxe2x88x92nC) and xcexdd=(ndxe2x88x921)/(nFxe2x88x92nC) wherein nd, nF, nC and ng are refractive indices with respect to d-line, F-line, C-line and g-line, respectively, and AgF and BgF are each a linear coefficient determined by two vitreous material types represented by glass code 511605 (available under the trade name of NSL7, Ohara Co., Ltd. with xcex8gF=0.5436 and xcexdd=60.49) and glass code 620363 (available under the trade name of PBM2, Ohara Co., Ltd. with xcex8gF=0.5828 and xcexdd=36.26);that is, AgF is 0.641462485 and BgF is xe2x88x920.001617829.
When the lower limit of 0.015 to condition (2) is not reached, short wavelength longitudinal chromatic aberrations remain under-corrected at the telephoto end, and so colors are likely to bleed out at the edges of a subject having a large luminance difference. Any inexpensive medium exceeding the upper limit of 0.1 is little available, and opposite chromatic aberrations occur above 0.1.
More preferably, conditions (2) and (3) should be reduced to
6.5 less than f1/L less than 16xe2x80x83xe2x80x83(1xe2x80x2) 
0.020 less than xcex94xcex8gF less than 0.08xe2x80x83xe2x80x83(2xe2x80x2) 
Most preferably, conditions (2) and (3) should be reduced to
7 less than f1/L less than 12xe2x80x83xe2x80x83(1xe2x80x3) 
0.025 less than xcex94xcex8gF less than 0.06xe2x80x83xe2x80x83(2xe2x80x3) 
According to the third embodiment of the present invention, there is provided a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of the zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of the zoom lens system along the optical axis during zooming from a wide-angle end to a telephoto end of the zoom lens system, has negative refracting power and comprises at least three negative lens elements while a positive lens element is located nearest to an image side of the second lens group, or three negative lens elements located nearest to an object side of the second lens group while a positive lens element is located on said image side or a negative lens element while two positive lens elements are located nearest to the image side of the second lens group, with any one of surfaces in the second lens group being defined by an aspheric surface, and a rear lens group having at least two movable subgroups and comprising a total of 6 to 11 lens elements inclusive or a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of the zoom lens system during zooming, a second lens group which moves toward an image side of the zoom lens system along the optical axis during zooming from a wide-angle end to a telephoto end of the zoom lens system and has negative refracting power and a rear lens group having at least two spacings variable during zooming, wherein the following condition is satisfied with respect to an amount of movement xcex94z1 of the first lens group from the wide-angle end to the telephoto end when the zoom lens system is focused on an object point at infinity and an amount of movement xcex94z2 of the second lens group from the wide-angle end to the telephoto end when the zoom lens system is focused on an object point at infinity:
3 less than (xcex94z2xe2x88x92xcex94z1)/L less than 9xe2x80x83xe2x80x83(3) 
where the movement of each lens group toward the image side is assumed to be positive and L is a diagonal length of an effective image pickup surface located in the vicinity of an image-formation plane.
For zooming from the wide-angle end to the telephoto end, the second lens group is relatively moved away from the first lens group, as already explained. Especially for a high-zoom-ratio zoom lens system, there must be a space large enough for the movement of the second lens group because that amount of movement is large. This is particularly true as the field angle of the zoom lens system becomes wide. As a result, the diameter of the first lens group often becomes too large. When the upper limit of 9 to condition (3) is exceeded, the diameter of the first lens group becomes too large and so the size of the zoom lens system becomes large. When the lower limit of 3 is not reached, there is an increased load of zooming on the rear lens group, which may result in large fluctuations of spherical aberrations upon zooming.
More preferably, condition (3) should be reduced to
3.2 less than (xcex94z2xe2x88x92xcex94z1)/L less than 8xe2x80x83xe2x80x83(3xe2x80x2) 
Most preferably, condition (3) should be reduced to
3.4 less than (xcex94z2xe2x88x92xcex94z1)/L less than 7xe2x80x83xe2x80x83(3xe2x80x3) 
When a zoom lens system has a wide-angle, high-zoom-ratio arrangement, the largest load is applied on the second lens group. In addition, even the magnitude of the diameter of the first lens group is determined by the power, amount of movement, and arrangement of the second lens group. In consideration of the diameter of the first lens group alone, it is favorable to locate the principal point of the second lens group as close to the object side as possible. Thus, it is preferable that the second lens group is constructed of a front subgroup having negative refracting power and a rear subgroup having positive refracting power. In this case, however, barrel distortion is likely to occur due to the wide-angle, high-zoom-ratio arrangement and difficulty is involved in making correction for astigmatism all over the zooming zone. These problems can substantially be eliminated if the second lens group is constructed of at least three negative lenses and a positive lens located nearest to the image side thereof, or three negative lenses located nearest to the object side thereof and a positive lens located on the image side, or a negative lens and two positive lenses located nearest to the image side thereof, with any one of the surfaces in the second lens group being defined by an aspheric surface.
When the number of lenses in the rear lens group is less than 6, severe conditions are added to correction of chromatic aberrations and spherical aberrations. When more than 11 lenses are used, on the other hand, the entire rear lens group becomes too thick to secure ample zooming space.
The rear lens group has a plurality of subgroups. In view of chromatic aberrations, spherical aberrations, coma and increased aperture, it is more preferable to construct the rear lens group of at least two subgroups having positive refracting power, wherein the subgroup located nearest to the image side thereof and having positive refracting power and the subgroup located nearest to the image side thereof and having positive refracting power are each composed of at least three lenses.
According to the fourth embodiment of the present invention, there is provided a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of the zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of the zoom lens system along the optical axis during zooming from a wide-angle end to a telephoto end of the zoom lens system, has negative refracting power and comprises at least three negative lens elements while a positive lens element is located nearest to an image side of the second lens group, or three negative lens elements located nearest to an object side of the second lens group while a positive lens element is located on said image side or a negative lens element while two positive lens elements are located nearest to the image side of the second lens group, with any one of surfaces in the second lens group being defined by an aspheric surface, and a rear lens group having at least two movable subgroups and comprising a total of 6 to 11 lens elements inclusive or a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of the zoom lens system during zooming, a second lens group which moves toward an image side of the zoom lens system along the optical axis during zooming from a wide-angle end to a telephoto end of the zoom lens system and has negative refracting power and a rear lens group having at least two spacings variable during zooming, wherein the following condition is satisfied with respect to an amount of movement xcex94z1 of the first lens group from the wide-angle end to the telephoto end when the zoom lens system is focused on an object point at infinity and an amount of movement xcex94z2 of the second lens group from the wide-angle end to the telephoto end when the zoom lens system is focused on an object point at infinity:
xe2x88x921.0 less than (xcex94z1/xcex94z2 less than 0.5 where xcex94z2 greater than 0xe2x80x83xe2x80x83(4) 
where the movement of each lens group toward the image side is assumed to be positive.
This is the condition for making a proper locus of an image point defined by a composite first-and-second lens group system upon zooming from the wide-angle end to the telephoto end. By this locus, the magnification-variable zone and focal length of the rear lens group are determined to some extent. When the upper limit of 0.5 to condition (4) is exceeded, the magnification of the rear lens group becomes low or the focal length of the rear lens group becomes long and, hence, the entire size of the zoom lens system tends to become large relative to the value of L. When the lower limit of xe2x88x921.0 is not reached, on the contrary, the entire size of the zoom lens system becomes small relative to the value of L. However, when the value of L is small and the F-number is small, it is difficult to make correction for spherical aberrations and comas.
It is acceptable to meet condition (4) and condition (3) simultaneously.
More preferably, condition (4) should be reduced to
xe2x88x920.9 less than (xcex94z1/xcex94z2 less than 0.4 where xcex94z2 greater than 0xe2x80x83xe2x80x83(4xe2x80x2) 
Most preferably, condition (4) should be reduced to
xe2x88x920.8 less than (xcex94z1/xcex94z2 less than 0.3 where xcex94z2 greater than 0xe2x80x83xe2x80x83(4xe2x80x3) 
According to the fifth embodiment of the present invention, there is provided a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of said zoom lens during zooming and has positive refracting power, a second lens group which moves toward an image side of said zoom lens system along said optical axis during zooming from a wide-angle end to a telephoto end of said zoom lens system and has negative refracting power, and a rear lens group having at least two movable subgroups or a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of said zoom lens during zooming and has positive refracting power, a second lens group which moves toward an image side of said zoom lens system along said optical axis during zooming from a wide-angle end to a telephoto end of said zoom lens system and has negative refracting power, and a rear lens group having at least two spacings variable during zooming, wherein said first lens group moves toward said image side in a convex reciprocation locus and an amount of movement xcex94z1WM of said first lens group from said wide-angle end to an intermediate focal length of said zoom lens system, given by fM(=√{square root over ( )}(fWxc2x7fT)), is positive where fW is a composite focal length of said zoom lens system when focused at said wide-angle end on an object point at infinity and fT is a composite focal length of said zoom lens system when focused at said telephoto end on an object point at infinity, with the proviso that the movement of said first lens group lens toward said image side is assumed to be positive and fM is the geometric mean of fW and fT. It is here noted that upon zooming from the wide-angle end to the telephoto end, the second lens group moves relatively away from the first lens group and the rear lens group moves in such a way that its principal point position goes off an image plane. It is also noted that the position of the image plane is kept constant.
When an electronic image pickup device or a viewing frame having a small value for L, the magnification of the rear lens group is particularly small or nearly one even at the telephoto end, because the ratio of the focal length of the first lens group to that of the zoom lens system becomes very large. At the same time, since the focal length of the rear lens group is longer than that of the second lens group, it is required that a locus of an image point defined by a composite first-and-second lens group system upon zooming from the wide-angle end to the telephoto end change considerably sharply toward the image side in the vicinity of the wide-angle end, and change considerably gently at the telephoto end. In other words, it is preferable that such a locus as mentioned above is taken by the first lens group.
According to the sixth embodiment of the present invention, there is a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of said zoom lens during zooming and has positive refracting power, a second lens group which moves toward an image side of said zoom lens system along said optical axis during zooming from a wide-angle end to a telephoto end of said zoom lens system and has negative refracting power, and a rear lens group having at least two movable subgroups or a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of said zoom lens during zooming and has positive refracting power, a second lens group which moves toward an image side of said zoom lens system along said optical axis during zooming from a wide-angle end to a telephoto end of said zoom lens system and has negative refracting power, and a rear lens group having at least two spacings variable during zooming, wherein said first lens group moves toward said image side in a convex reciprocation locus and only the aforesaid condition (4) or both conditions (3) and (4) are satisfied.
In this embodiment, too, the effects mentioned with reference to the fourth and fifth embodiments are obtainable.
According to the seventh embodiment of the present invention, there is provided a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of said zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of said zoom lens system along said optical axis during zooming from a wide-angle end to a telephoto end of said zoom lens system, has negative refracting power and comprises at least three negative lenses while a positive lens is located nearest to said image side, or three negative lenses located nearest to said object side while a positive lens is located on said image side or a negative lens while two positive lenses are located nearest to said image side, with any one of surfaces in said second lens group being defined by an aspheric surface, and a rear lens group having at least two movable subgroups and comprising a total of 6 to 11 lens elements inclusive, or a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of said zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of said zoom lens system along said optical axis during zooming from a wide-angle end to a telephoto end of said zoom lens system and has negative refracting power and a rear lens group having at least two spacings variable during zooming.
When a zoom lens system has a wide-angle, high-zoom-ratio arrangement, the largest load is applied on the second lens group. In addition, even the magnitude of the diameter of the first lens group is determined by the power, amount of movement, and arrangement of the second lens group. In consideration of the diameter of the first lens group alone, it is favorable to locate the principal point of the second lens group as close to the object side as possible. Thus, it is preferable to locate a positive lens nearest to the image side of the second lens group. In this case, however, barrel distortion is likely to occur due to the wide-angle, high-zoom-ratio arrangement and difficulty is involved in making correction for astigmatism all over the zooming zone. These problems can substantially be eliminated if the second lens group is constructed of at least three negative lenses, wherein at least one surface is formed by an aspheric surface. In particular, it is preferable that the aspheric surface is of such a shape that off and off the center of the aspheric surface, its divergence becomes weaker or its convergence becomes stronger as compared with its longitudinal curvature. Even when the second lens group is constructed of three negative lenses located nearest to the object side thereof with a positive lens located on the image side thereof or constructed of a negative lens with two positive lenses located nearest to the image side thereof, similar effects are obtainable as already mentioned.
Furthermore in this embodiment, the following conditions should preferably be satisfied with respect to a xcex22T/xcex22W ratio xcex94xcex22 where xcex22T is the magnification of the second lens group at the telephoto end and xcex22W is the magnification of the second lens group at the wide-angle end when the zoom lens system is focused on an object point at infinity and the focal length f2 of the second lens group.
0.3 less than log(xcex94xcex22)/log(xcex3) less than 0.8xe2x80x83xe2x80x83(5) 
5 less than xcex3 less than 15xe2x80x83xe2x80x83(6) 
Here xcex3 is the zoom ratio of the zoom lens system from the wide-angle end to the telephoto end.
When a zoom lens system has a wide-angle, high-zoom-ratio arrangement, the largest load is applied on the second lens group, as already mentioned. In addition, even the magnitude of the diameter of the first lens group is determined by the power, amount of movement, and arrangement of the second lens group. It is thus preferable to allocate the zooming function to the rear lens group as much as possible. Condition (5) is provided to define the proportion of the zoom ratio of the second lens group all over the zooming zone. When the upper limit of 0.8 is exceed, the load of the zooming function on the second lens group becomes too large to make correction for the aforesaid off-axis aberrations and reduce the diameter of the first lens group. When the lower limit of 0.3 is not reached, on the contrary, the load of the zooming function on the rear lens group becomes too large and, hence, it is difficult to achieve large aperture because spherical aberrations, coma and so on become instable all over the zooming zone. Condition (6) represents the zoom ratio range wherein condition (5) is effective. Any departure from this range causes condition (5) to be ineffective. In other words, when the upper limit of 15 to condition (6) is exceeded, it is preferable to reduce the degree of allocation of the zooming function to the second lens group to below the lower limit to condition (5). When the lower limit of 5 is not reached, on the other hand, it is acceptable to increase the degree of allocation of the zooming function to the second lens group to greater than the upper limit to condition (5) because influences of aberrations diminish. However, any sufficient zoom ratio is not obtainable.
More preferably, the aforesaid conditions should be
0.35 less than log(xcex94xcex22)/log(xcex3) less than 0.65xe2x80x83xe2x80x83(5xe2x80x2) 
9 less than xcex3 less than 15xe2x80x83xe2x80x83(6xe2x80x2) 
or
0.5 less than log(xcex94xcex22)/log(xcex3) less than 0.8xe2x80x83xe2x80x83(5xe2x80x3) 
5 less than xcex3 less than 9xe2x80x83xe2x80x83(6xe2x80x3) 
According to the eighth embodiment of the present invention, there is provided a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of said zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of said zoom lens system along said optical axis during zooming from a wide-angle end to a telephoto end of said zoom lens system, has negative refracting power and comprises at least three negative lenses while a positive lens is located nearest to said image side, or three negative lenses located nearest to said object side while a positive lens is located on said image side or a negative lens while two positive lenses are located nearest to said image side, with any one of surfaces in said second lens group being defined by an aspheric surface, and a rear lens group having at least two movable subgroups and comprising a total of 6 to 11 lens elements inclusive, or a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of said zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of said zoom lens system along said optical axis during zooming from a wide-angle end to a telephoto end of said zoom lens system and has negative refracting power and a rear lens group having at least two spacings variable during zooming, wherein the following condition is satisfied with respect to the composite magnification xcex2rW of the rear lens group when the zoom lens system is focused at the wide-angle end on an object point at infinity.
xe2x88x920.6 less than xcex2rW less than xe2x88x920.1xe2x80x83xe2x80x83(7) 
As already mentioned, when an image pickup device or a film viewing frame having a small value for L (the diagonal length of an effective image pickup surface) is used, the ratio of the focal length of the first lens group to that of the zoom lens system becomes very large. For instance, this is because the simple proportional coefficient multiple of an optical system for 135 mm format or APS format cannot be physically applied to mechanical construction or lens machining. For this reason, it is impossible to reduce the focal length of each lens group, and especially the composite focal length of the first and second lens groups. In other words, the magnification of the rear lens group must be smaller than that of an optical system for the aforesaid formats. When the lower limit of xe2x88x920.6 to condition (7) is not reached, the focal length of the composite first-and-second lens group system tends to become short and, hence, the edge thickness, center thickness and air space of each lens tend to become extremely small. An attempt to secure these make the Petzval sum of the optical system negative and, at the same time, renders it difficult to secure off-axis aberrations such as distortion, astigmatism and coma all over the zooming zone. When the upper limit of xe2x88x920.1 is exceeded, the lens system tends to become huge.
It is preferable that the aforesaid rear lens group is composed of at least three subgroups, each having a variable axial relative distance, and three such subgroups have positive, negative, and positive power in order from the object side of the rear lens group.
Alternatively, it is preferable that the rear lens group is composed of a plurality of subgroups, each having a variable axial relative distance, and all subgroups in the rear lens group have each at least one doublet component. Still alternatively, it is preferable that the rear lens group is composed of at least three subgroups, each having a variable axial relative distance and all subgroups in the rear lens group have each at least one doublet component.
It is more preferable that when
9 less than xcex3 less than 15xe2x80x83xe2x80x83(6xe2x80x2),
xe2x88x920.5 less than xcex2rW less than xe2x88x920.1xe2x80x83xe2x80x83(7xe2x80x2),
or when
5 less than xcex3 less than 9xe2x80x83xe2x80x83(6xe2x80x3),
xe2x88x920.6 less than xcex2rW less than xe2x88x920.2xe2x80x83xe2x80x83(7xe2x80x3).
According to the ninth embodiment of the present invention, there is provided a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of said zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of said zoom lens system along said optical axis during zooming from a wide-angle end to a telephoto end of said zoom lens system, has negative refracting power and comprises at least three negative lenses while a positive lens is located nearest to said image side, or three negative lenses located nearest to said object side while a positive lens is located on said image side or a negative lens while two positive lenses are located nearest to said image side, with any one of surfaces in said second lens group being defined by an aspheric surface, and a rear lens group having at least two movable subgroups and comprising a total of 6 to 11 lens elements inclusive, or a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of said zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of said zoom lens system along said optical axis during zooming from a wide-angle end to a telephoto end of said zoom lens system and has negative refracting power and a rear lens group having at least two spacings variable during zooming, wherein focusing is effected by any one of subgroups located nearer to an image side of said rear lens group than a positive subgroup of subgroups having negative magnification, said positive subgroup located nearest to an object side of said rear lens group, and the following condition is satisfied with respect to a magnification xcex2RRW of said positive subgroup located nearest to the image side of said rear lens group when said zoom lens system is focused at said wide-angle end on an object point at infinity:
xe2x88x920.4 less than xcex2RRW less than 0.9xe2x80x83xe2x80x83(8) 
In the present invention, focusing is effected by moving a subgroup or subgroups in the rear lens group on the optical axis, and zooming is effected by the second lens group and the rear lens group. Actually, however, only the subgroup of a plurality of subgroups constituting the rear lens group, which subgroup has positive refracting power and negative magnification and is located nearest to the object side of the rear lens group, contributes to zooming. Other subgroups are designed to have magnifications far away from xe2x88x921, so that focusing can be done by one or more of the subgroups. It is particularly preferable to effect focusing with a positive subgroup located nearest to the image side of the rear lens group, because there are little fluctuations of aberrations with focusing. Condition (8) is provided to define the magnification xcex2RRW of the positive subgroup located nearest to the image side of the rear lens group. Falling below the lower limit of xe2x88x920.4 is not preferable because of increased fluctuations of the paraxial amount and the amount of aberrations. Exceeding the upper limit of 0.9 is again not preferable because the amount of movement of the focusing subgroup becomes too large and so this subgroup tends to interfere with the adjacent subgroup before focusing is achieved from an object point at infinity to a close-up object point.
It is preferable that focusing is effected by the positive subgroup located nearest to the image side of the rear lens group and/or a negative subgroup located on the object side of the rear lens group, because fluctuations of aberrations with focusing can be so reduced that proper focusing and proper sensitivity can be obtained.
More preferably, condition (8) should be reduced to
xe2x88x920.3 less than xcex2RRW less than 0.8xe2x80x83xe2x80x83(8xe2x80x3) 
Most preferably, condition (8) should be reduced to
xe2x88x920.2 less than xcex2RRW less than 0.7xe2x80x83xe2x80x83(8xe2x80x3) 
According to the tenth embodiment of the present invention, there is provided a a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of said zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of said zoom lens system along said optical axis during zooming from a wide-angle end to a telephoto end of said zoom lens system, has negative refracting power and comprises at least three negative lenses while a positive lens is located nearest to said image side, or three negative lenses located nearest to said object side while a positive lens is located on said image side or a negative lens while two positive lenses are located nearest to said image side, with any one of surfaces in said second lens group being defined by an aspheric surface, and a rear lens group having at least two movable subgroups and comprising a total of 6 to 11 lens elements inclusive, or a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of said zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of said zoom lens system along said optical axis during zooming from a wide-angle end to a telephoto end of said zoom lens system and has negative refracting power and a rear lens group having at least two spacings variable during zooming, wherein the following conditions are satisfied with respect to an amount of movement xcex94zRF of a subgroup of said subgroups in said rear lens group, said subgroup having positive refracting power and located nearest to an object side of said rear lens group, from said wide-angle end to said telephoto end when said zoom lens system is focused on an object point at infinity and an amount of movement xcex94zRR of a positive subgroup located nearest to an image side of said rear lens group when said zoom lens system is focused on an object point at infinity:
xe2x88x920.4 less than xcex94zRR/xcex94zRF less than 0.8xe2x80x83xe2x80x83(9) 
0.3 less than |xcex94zRF|/L less than 4.0xe2x80x83xe2x80x83(10) 
where L is a diagonal length of an effective image pickup surface located in the vicinity of an image-formation plane.
Of the subgroups constituting the rear lens group, the positive subgroup located nearest to the object side of the rear lens group contributes actually to zooming. Consequently, this subgroup moves monotonously toward the object side of the zoom lens system from the wide-angle end to the telephoto end thereof. Other subgroups have magnifications far away from xe2x88x921, and move or act substantially to make correction for displacements of focusing positions due to zooming and aberrations. On the other hand, as the positive subgroup located nearest to the image side of the rear lens group moves toward the object side of the zoom lens system than required, the position of an exit pupil comes close to the image plane. For this reason, when an electronic image pickup device is used, shading is likely to occur. When the upper limit of 0.8 to condition (9) is exceeded, the exit pupil comes close to the image plane on the telephoto side, and so the angle of light rays incident on the perimeter of a screen becomes too large. When the lower limit of xe2x88x920.4 is not reached, the total thickness of the rear lens group increases and so the overall size of the optical system becomes large. When the upper limit of 4.0 to condition (10) is exceeded, it is likely that the overall length of the optical system becomes long or fluctuations of aberrations with zooming become noticeable. When the lower limit of 0.3 is not reached, the diameter of the first lens group is likely to become large. These are true even when at least one subgroup is placed midway between the aforesaid two positive subgroups. Especially when that subgroup is a negative one, it is preferable to satisfy
xe2x88x922 less than xcex94zRN/L less than 1xe2x80x83xe2x80x83(11) 
Here xcex94zRN is the amount of movement of the negative subgroup from the wide-angle end to the telephoto end when the zoom lens system is focused on an object point at infinity. When the lower limit of xe2x88x922 to this condition is not reached, the total thickness of the rear lens group increase and so the overall size of the optical system becomes large. When the upper limit of 1 is exceeded, it is likely that the subgroups interfere during focusing on a nearby object point at the telephoto end. This holds true even when a negative subgroup is located on the object side with respect to the aforesaid positive subgroups and on the image side with respect to the second lens group.
More preferably, the following conditions should be satisfied independently or simultaneously.
xe2x88x920.3 less than xcex94zRR/xcex94zRF less than 0.7xe2x80x83xe2x80x83(9xe2x80x2) 
0.5 less than |xcex94zRF|/L less than 3.5xe2x80x83xe2x80x83(10xe2x80x2) 
xe2x88x921.5 less than xcex94zRN/L less than 0.7xe2x80x83xe2x80x83(11xe2x80x2) 
Most preferably, the following conditions should be satisfied independently or simultaneously.
xe2x88x920.2 less than xcex94zRR/xcex94zRF less than 0.6xe2x80x83xe2x80x83(9xe2x80x3) 
0.7 less than |xcex94zRF/L less than 3.0xe2x80x83xe2x80x83(10xe2x80x3) 
xe2x88x921 less than xcex94zRN/L less than 0.5xe2x80x83xe2x80x83(11xe2x80x3) 
It is also preferable that the positive subgroup located nearest to the object side of the rear lens group has negative magnification in view of its contribution to zooming.
According to the eleventh embodiment of the present invention, there is provided a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of said zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of said zoom lens system along said optical axis during zooming from a wide-angle end to a telephoto end of said zoom lens system, has negative refracting power and comprises at least three negative lenses while a positive lens is located nearest to said image side, or three negative lenses located nearest to said object side while a positive lens is located on said image side or a negative lens while two positive lenses are located nearest to said image side, with any one of surfaces in said second lens group being defined by an aspheric surface, and a rear lens group having at least two movable subgroups and comprising a total of 6 to 11 lenses inclusive, said rear lens group comprising a subgroup having positive refracting power and negative magnification and a positive subgroup located nearest to an image side of said rear lens group which vary in relative positions thereof during zooming, or a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of said zoom lens during zooming and has positive refracting power, a second lens group which moves toward an image side of said zoom lens system along said optical axis during zooming from a wide-angle end to a telephoto end of said zoom lens system and has negative refracting power and a rear lens group having a plurality of subgroups, said rear lens group comprising a subgroup having positive refracting power and negative magnification and a positive subgroup located nearest to an image side of said rear lens group with a negative subgroup located between said two positive subgroup, while said three subgroup vary in relative positions thereof during zooming, wherein said two positive subgroups have each at least one doublet component, at least one aspheric surface and at least one lens formed of a vitreous material with xcexd greater than 80 where xcexd is an Abbe constant. Since the chromatic aberrations, spherical aberrations and comas of each lens group are in good condition, satisfactory images can be obtained from the wide-angle end to the telephoto end. It is here preferable that the negative subgroup located midway between the two positive subgroups includes a doublet.
According to the twelfth embodiment of the present invention, there is provided a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of said zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of said zoom lens system along said optical axis during zooming from a wide-angle end to a telephoto end of said zoom lens system, has negative refracting power and comprises at least three negative lenses while a positive lens is located nearest to said image side, or three negative lenses located nearest to said object side while a positive lens is located on said image side or a negative lens while two positive lenses are located nearest to said image side, with any one of surfaces in said second lens group being defined by an aspheric surface, and a rear lens group having at least two movable positive subgroups and comprising a total of 7 to 11 lenses inclusive, or a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of said zoom lens during zooming and has positive refracting power, a second lens group which moves toward an image side of said zoom lens system along said optical axis during zooming from a wide-angle end to a telephoto end of said zoom lens system and has negative refracting power and a rear lens group having at least three spacings variable during zooming, wherein a subgroup located nearest to an object side of said rear lens group has negative refracting power.
In the zoom lens system according to the present invention, when a finder optical path-splitting member is inserted between the subgroup located nearest to the image side of the rear lens group and the image lance, a long back focus is needed. In other words, an attempt to forcibly ample back focus makes the Petzval sum of the zoom lens system likely to become negative. It is thus preferable that a negative lens subgroup is located nearest to the object side of the rear lens group. It is here noted that the negative subgroup located nearest to the object side of the rear lens group may be made up of one lens component or fixed in the vicinity of a stop. By the xe2x80x9clens componentxe2x80x9d used herein is intended a lens with no air separation between the object-side surface and the image-side surface thereof, which are in contact with air, or specifically a single lens or a doublet.
It is preferable that a negative subgroup and an aperture stop are located on the object side with respect to the two positive subgroup and on the image side with respect to the second lens group, with a spacing being at most three times as large as the thickness of that negative subgroup on the optical axis of the zoom lens system.
When a subgroup having negative refracting power is located nearest to the object side of the rear lens group, it is preferable to construct the rear lens group of seven or more lenses in all.
In the eleventh embodiment of the invention, it is preferable that the zoom lens system comprises, in order from an object side thereof, a first lens group which is movable along an optical axis thereof during zooming and positive refracting power, a second lens group which is movable along the optical axis and has negative refracting power, and a rear lens group located subsequent thereto and having variable refracting power, while at least one of the following three conditions is satisfied.
2.0 less than FBW/fW less than 5.0xe2x80x83xe2x80x83(12) 
1.4 less than FW less than 3.5xe2x80x83xe2x80x83(13) 
2 less than ENP/L less than 5xe2x80x83xe2x80x83(14) 
Here FBW is the back focus (calculated on an air basis) when the zoom lens system is focused at the wide-angle end on an object point at infinity, FW is the minimum F-number when the zoom lens system is focused at the wide-angle end on an object point at infinity, and ENP is the position of an entrance pupil at the wide-angle end.
The present invention is found to be effective for lens systems that satisfy one of these conditions. In particular, the present invention is best suited for image pickup systems using electronic image pickup devices. Especially when the present invention is applied to an image-formation optical system for phototaking systems (cameras, video movies, etc.) including a high-pixel image pickup device with a pixel interval a represented by
1.0xc3x9710xe2x88x924xc3x97L less than a less than 6.0xc3x9710xe2x88x924xc3x97L (mm) 
it is possible to achieve an image pickup system making effective use of the image quality of a high-pixel arrangement.
Two or more of the conditions mentioned above with reference to the present zoom lens system should preferably be satisfied simultaneously. More preferably, two or more of the requirements for the present invention should be satisfied at the same time. The more the number of the requirements met, the better the results are.
In each of the embodiments of the present invention, it is preferable that the second lens group comprises at least three negative lenses while a positive lens is located nearest to said image side, or three negative lenses located nearest to said object side while a positive lens is located on said image side or a negative lens while two positive lenses are located nearest to said image side, with any one of surfaces in said second lens group being defined by an aspheric surface. When the rear lens group has at least two spacings variable during zooming, it is preferable that the rear lens group is made up of 7 to 11 lenses in all. More preferably, the rear lens group is made up of 7 to 9 lenses inclusive in all while two aspheric surfaces are used, because an arrangement favorable in view of size is achievable while high image-formation capability is maintained.
By the combined use of two or more of the aforesaid embodiments, it is possible to obtain ever higher effects.
Still other objects and advantages of the invention will be in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.